The present invention relates to a noise canceling circuit for canceling a noise that enters into a clock input terminal, or the like, irrespective of a variation in production.
FIG. 6 is a circuit diagram showing a configurative example of a noise canceling circuit using an RC filter in the prior art. In FIG. 6, reference numerals 1 to 4 are inverters, R1 is a resistor, and C1 is a capacitor. Now, when a signal IN containing a noise NZ shown in FIG. 7A is input into an input terminal, a signal ND2 at a connection point between the resistor R1 and the capacitor C1 appears as shown in FIG. 7B and an output signal ND3 of the inverter 2 and an output signal OUT of the inverter 4 appear as shown in FIGS. 7C and 7D, respectively. As apparent from this Figure, if a width of the noise NZ exceeds a predetermined value, such noise cannot be absorbed by an RC filter and the noise appears on the output signal OUT. This can be improved by constructing the inverter 2 as a Schmidt circuit.
FIGS. 8A to 8D are operating waveform diagrams when the Schmidt circuit is used. The output signal ND2 of the RC filter is given as shown by a thin line in FIGS. 8B to 8D in response to the input signal IN containing the noise NZ shown in FIG. 8A, and the output signal OUT is given as shown by a thick line in the same Figure in response to threshold levels VIL, VIH of the Schmidt circuit. In other words, if the threshold levels VIL is low, the output signal OUT rises later regardless of the noise NZ, as shown in FIG. 8B. If both the threshold levels VIL, VIH are higher than those in the case of FIG. 8B, the output signal OUT rises when the signal ND2 crosses the threshold levels VIL, as shown in FIG. 8C. In this manner, if the Schmidt circuit is used, the influence of the noise can also be suppressed by the Schmidt circuit. However, if the threshold levels VIL is relatively high and the threshold levels VIH is low, it is possible that the noise responding to the noise NZ appears in the output signal OUT, as shown in FIG. 8D.
Patent Literature 1 discloses the circuit in which the hysteresis input circuit is implemented by the internal circuit without the external circuit and the noise is canceled by this hysteresis input circuit. However, even though the hysteresis characteristic is provided to the input circuit, in some cases the noise cannot be canceled according to the type of the noise. In the circuit disclosed in Patent Literature 2, the hysteresis characteristic is provided to the input circuit and also the delay characteristic is provided to the feedback loop by applying the positive feedback from the output end to the input end. However, this circuit can cancel the narrow noise, but such circuit has such a shortcoming that it cannot cancel the noise whose width is in excess of a predetermined value.
Patent Literature 3 discloses the noise canceling circuit in which the input stage is constructed by the Schmidt circuit with the hysteresis characteristic. However, this circuit has such a shortcoming that it does not operate when the input signal does not have a width that is in excess of a predetermined value.
Patent Literature 1
JP-B-3-30323
Patent Literature 2
JP-A-59-172826
Patent Literature 3
JP-B-1-29094